Stable Boundary Layer in Tropical Cyclones: Observations and Coupled Modeling of Its Impact on Storm Structure

Tropical cyclone (TC) boundary layer (BL) plays an important role on the storm structure and intensity. It connects the TC convection in rainbands and eyewall to the underlying ocean, which affects the air-sea fluxes and is a key component of the air-sea coupling in TCs. A high-resolution coupled modeling study has suggested that the formation of a stable boundary layer (SBL) over a TC-induced near-storm (in situ) cold-wake may enhance the near-surface inflow into the TC inner core and eyewall and strengthen the TC intensity. However, observations of these properties have been lacking until the recent field campaign the Impact of Typhoons on the Ocean in the Pacific (ITOP) over the west Pacific in 2010. Coupled atmospheric and oceanic observations, including aircraft deployed GPS dropsondes and AXBTs, obtained in Typhoon Fanapi (2010) show a clear SBL over a relatively strong and persistent cold wake. It is found that when the warm air flowing over the cooler sea surface, the boundary layer becomes stable in contrast to near neutral or unstable boundary elsewhere in Fanapi. There is a reduction in surface enthalpy fluxes and a negative (downward from air to sea) sensible heat fluxes in SBL over the cold wake. The region downstream of the cold wake and SBL shows an enhanced air-sea enthalpy flux where relatively cooler air over warm SST is observed. This study presents a detailed observation of SBL in Typhoon Fanapi during ITOP. A high-resolution, fully coupled atmosphere-ocean model is used to provide further insights as how the SBL influence TC structure and intensity in TCs.